muted this structure to a locomotory blade 

 (as modem whales do in shortening the 

 tail and evolving a prominent horizontal 

 fluke as the animal's major means of 

 propulsion). Unfortunately, no pelvic 

 bones have been found, but most elements 

 of a large, powerful hind leg were recov- 

 ered — including a complete femur, parts 

 of the tibia and fibula, an astragalus (ankle 

 bone), three metatarsal (foot) bones, and 

 several phalanges. To quote the authors: 

 "The feet are enormous." The fourth 

 metatarsal, for example, is nearly six 

 inches long, and the associated toe almost 

 seven inches in length. Interestingly, the 

 last phalanx of each toe ends in a small 

 hoof, as in terrestrial mesonychid ances- 

 tors. 



This new bounty of information allows 

 us to infer not only the form of this transi- 

 tional whale but also, with good confi- 

 dence, its intermediary style of locomo- 

 tion and mode of life (an impossibility 

 with the first three cases, for Pakicetus is 

 only a skull, Basilosaurus had already 

 crossed the bridge, and Indocetus is too 

 fragmentary). The forelimbs, smaller than 

 the hind and limited in motion, were 

 "probably used in maneuvering and steer- 

 ing while swimming, as in extant 

 cetaceans ["modem whales" in ordinary 

 language], and they lacked a major 

 propulsive force in water." 



Modem whales move through the water 

 by powerful beats of their horizontal tail 

 fluke — a motion made possible by strong 

 undulation of a flexible rear spinal col- 

 umn. Ambulocetus had not yet evolved a 

 tail fluke, but the spine had requisite flexi- 

 bility. Thewissen and colleagues write: 

 ''Ambulocetus swam by means of 

 dorsoventral [back-to-belly] undulations 



of its vertebral column, as evidenced by 

 the shape of the lumbar [lower back] ver- 

 tebra." These undulations then functioned 

 with (and powered) the paddling oiAmbu- 

 locetus's large feet, which provided the 

 major propulsive force in swimming. 

 Thewissen et al. conclude their article: 



Like modern cetaceans — it swam by mov- 

 ing its spine up and down, but like seals, the 

 main propulsive surface was provided by its 

 feet. As such, Ambulocetus represents a crit- 

 ical intermediate between land mammals 

 and marine cetaceans. 



Ambulocetus was no ballet dancer on 

 land, but we have no reason to judge this 

 creature as any less efficient than modem 

 sea lions, which do manage, however inel- 

 egantly. Forelimbs may have been held 

 out to the sides, largely for stability, with 

 forward motion supplied primarily by ex- 

 tension of the back and consequent flexing 

 of the hind limbs — again, rather like sea 

 hons. 



Verdict: Greedy paleontologists, used to 

 working with fragments in reconstmcting 

 wholes, always want more (some pelvic 

 bones would be nice, for starters), but if 

 you had given me both a blank piece of 

 paper and a blank check, I could not have 

 drawn you a theoretical intermediate any 

 better or more convincing than Ambuloce- 

 tus. Those dogmatists who by verbal trick- 

 ery can make white black, and black 

 white, will never be convinced of any- 

 thing, but Ambulocetus is the very animal 

 that they proclaimed impossible in theory. 



Some discoveries in science are excit- 

 ing because they revise or reverse previ- 

 ous expectations; others because they af- 

 firm with elegance something well 

 suspected, but previously undocumented. 

 Our four-case story, culminating in Ambu- 



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locetus, falls into this second category. 

 This sequential discovery of picture-per- 

 fect intermediacy in the evolution of 

 whales stands as a triumph in the history 

 of paleontology. I cannot imagine a better 

 tale for popular presentation of science or 

 a more satisfying, and intellectually based, 

 political victory over lingering creationist 

 opposition. 



Still, I must confess that tiiis part of the 

 tale does not intrigue me most as a scien- 

 tist and evolutionary biologist. I don't 

 mean to sound jaded or dogmatic, but Am- 

 bulocetus is so close to our expectation for 

 a transitional form that its discovery could 

 not provide a professional paleontologist 

 with the greatest of all pleasures in sci- 

 ence — surprise. As a public illustration, 

 and as a sociopolitical victory, transitional 

 whales may be the story of the decade, but 

 paleontologists didn't doubt their exis- 

 tence or feel that a central theory would 

 collapse if their absence continued. We 

 love to place flesh upon our expectations 

 (or put bones under them, to be more hter- 

 ally correct), but this kind of delight takes 

 second place to the inteUectual jolting of 

 surprise. 



I therefore find myself far more in- 

 trigued by another aspect of Ambulocetus 

 that has not received much attention, ei- 

 ther in technical or popular reports. For the 

 anatomy of this transitional form illus- 

 trates a different and vital principle in evo- 

 lutionary theory — one rarely discussed or 

 even explicitly formulated, but central to 

 any understanding of nature's fascinating 

 historical complexity. 



hi our Darwinian traditions, we focus 

 too narrowly on the adaptive nature of or- 

 ganic form, and too little on the quirks and 

 oddities encoded into every animal by his- 

 tory. We are so overwhelmed — as well we 

 should be — by the intticacy in aerody- 

 namic optmiality of a bird's wing or by the 

 uncannily precise protective resemblance 

 evolved by certain butterflies that mimic 

 dead leaves. We do not ask often enough 

 why natural selection has honed in upon 

 this particular optimum — and not another 

 among a set of unrealized alternatives. In 

 other words, we are dazzled by good de- 

 sign and therefore stop our inquiry too 

 soon when we have answered "how does 

 this work so well?" — when we should also 

 be asking the historian's questions: "why 

 this and not thatT or "why this over here 

 and that in a related creature Uving else- 

 where?" 



To give the cardinal example from 

 seagoing mammals: the two fully marine 

 orders Sirenia and Cetacea swim by beat- 



14 Natural History 5/94 



